Heretofore, with few exceptions, reciprocating engines have been designed to operate at a fixed compression ratio, which is the ratio of the maximum volume to minimum volume in an engine cylinder during each compression/expansion cycle. The minimum volume occurs with the piston at "Top Dead Center" (TDC) and the maximum volume at "Bottom Dead Center" (BDC). An Otto cycle engine is designed to operate with the highest value of compression ratio that will preclude knocking at full throttle. Higher values of compression ratio help increase thermal efficiency according to the well known expression, EQU E=1-(1/r).sup.k-1 ( 1)
where
E denotes thermal efficiency PA1 r denotes compression ratio, and PA1 k is equal to the ratio of specific heats cp and cv at constant pressure and constant volume, and, in this instance, has a value close to 1.4.
While the efficiency improvement predicted by equation (1) may not quite be reached in practice, the expression serves to show the substantial influence of compression ratio upon thermal efficiency.
Higher values of compression ratio do not necessarily imply higher values of pressure and temperature. As pressure and temperature are functions of both volume and mass of the gas in-taken, while compression ratio is a ratio of volumes, temperature and pressure may be kept below a particular level by an increase in the size of the cylinder for fixed mass of the gas, or by a decrease in the mass of gas for same size of cylinder. It can be shown that for a particular amount of fuel mixture in-taken at full throttle the compression ratio could be increased from a value r1 to a higher value r2 without increasing maximum temperature and pressure, if the size of the cylinder is increased by a factor equal to (r2-1)/(r1-1). Conversely, for same size cylinder the pressure and temperature will not exceed the previous levels if the throttle is adjusted to permit, in the instance of the higher compression ratio r2, only a (r1-1)/(r2-1) portion of gas of what would have entered the cylinder at full throttle at compression ratio r1. These relationships are based on purely geometrical considerations and do not reflect the effect of other parameters, such as spark advance, rates of combustion and heat transfers. However, detailed computer simulation has shown that even after all the factors are taken into account the dominant relationship in part throttle operation still remains approximately equal to the (r1-1)/(r2-1) relationship.
Higher temperatures and pressure values than those now prevailing in gasoline engines are undesirable as they produce knocking and more NOx 20 pollutants.
It is understood by every driver that the power demand on automotive engines continuously varies with driving conditions. Use of the accelerator pedal helps in throttling more or less fuel to provide for the power demand. It is also known to the automobile drivers that seldom do they have to keep the accelerator pedal all the way down to the floor and that on the average they drive at part throttle. The automobiles therefore could be more fuel efficient if they were to be operated with a compression ratio value that could continuously vary with power demand to maintain maximum, temperature and pressure just below knocking levels.
This reasoning was used during the early sixties by the British Internal Combustion Engine Research Association (BICERA) to design a piston that could self adjust the compression ration on the basis of the pressure in the combustion chamber. This was implemented by providing a double piston, the actual piston telescoping over an internal piston. The extend of the telescoping, corresponding to higher compression ratio values, was accomplished by the amount of oil which was introduced during the intake stroke in the space between the two pistons and did not escape back to the reservoir. Higher pressures squeezed more oil from in between the two pistons to the reservoir, causing the outer piston to withdraw away from the cylinder head, thereby reducing the compression ratio. To quote from Air-Cooled Automotive Engines, by Julius Mackerie, John Willey & Sons, 1972 p. 372, "In the Continental engine, performance was increased by 50% without increasing the maximum pressure acting on the piston. At the same time conditions were improved for cold starting and for multi-fuel operation at increased compression ratio under partial load."
The objections to the BICERA approach for controlling the compression ratio, upon which the present invention provides improvements, are:
(1) As each cylinder functions independently from the other cylinders, wear, inaccuracies and fowling of the internal valves by hard particles in the oil, can generate unbalance, causing each cylinder to operate at a different compression ratio.
(2) The BICERA design provides only an approximate control of the compression ratio; while in today's computerized automobile engines with electronic injection and detailed information of the combustion process, it is desirable to have a more accurate control of the compression ratio.
A detailed computer simulation of the thermodynamic processes during each engine cycle revealed that while the capability of varying the compression ratio in reciprocating gasoline engines can provide an increase in the indicated thermal efficiency from about 30% to 35% for a change in compression ratio from 8.5:1 to 20:1, much greater increases are possible with the simultaneous use of ceramics. Thus an engine having its piston and cylinder head covered with ceramic and operated at 89% of open throttle and a fixed compression ratio of 8.5:1 would yield an indicated efficiency of about 34%; while the variable compression ratio its efficiency can go as high as 43.6% at a compression ratio of 20:1 and 40% throttle. During this type of operation the throttle opening is being continuously adjusting according to the (r1-1)/(r2-1) law, with r1 having been assigned a value equal to 8.5:1. This is extremely encouraging, especially when considering that a totally ceramic engine with much greater risks, turns out to yield only about 1.5% indicated efficiency. The facility of adjusting the compression ratio, therefore, can provide in this instance, a working performance improvement of 27% in indicated efficiency, with minor changes in both pressure and temperature, at part throttle.